1. Field of the Invention
The present invention relates to a capacitive fingerprint sensor and the panel thereof, and more particularly to a capacitive fingerprint sensor using a plurality of transistors and the panel thereof.
2. Description of the Related Art
A fingerprint sensor is a sensor for recognizing a pattern of a human fingerprint and providing reliable personal identification. The fingerprint sensor is also widely used in portable products such as mobile phones or notebooks in order to achieve security of personal confidential information.
FIGS. 1A and 1B show a hint diagram about the relationship between the human finger and a substrate panel and the corresponding equivalent diagram. Generally, the fingerprint sensor could be implemented in a chip or embedded in an image panel. If the fingerprint sensor is implemented in a chip, the number 13 in FIG. 1A represents a passivation layer, which acts as the dielectric layer of the capacitor Cd. If the fingerprint sensor is embedded in an image panel, the number 11 in FIG. 1A represents ITO layer, and number 13 represents glass and thin films, such as color filter, polarizer and etc. The following description takes the fingerprint sensor embedded in the image panel as an example. In FIG. 1A, a glass 13, a top metal plate 11 and a substrate 12 are combined in series, and the glass 13 is the place where the human finger will touch. Normally, a capacitor Cd exists in the glass 13, and a capacitor Cp exists between the top metal plate 11 and the substrate 12. In FIG. 1B, the surface of the human finger has ridges and valleys, such as the ridge 14 and the valley 15. The valley 15 has a distance d2 away from the glass 13 with a thickness d1. Based on the structure, an additional capacitor C2 exists between the valley of the human finger and the surface of the glass 13. The equivalent capacitance of the ridge capacitor CFR is related to C1, and the equivalence capacitance of the valley capacitors CFV is related to C1//C2. The capacitances of C1 and C1//C2 are listed as follows:
            C      1        =                            ɛ          1                ⁢        A                    d        1              ,                    and        ⁢                                  ⁢                  C          1                    //              C        2              =          1                                    d            1                                              ɛ              1                        ⁢            A                          +                              d            2                                              ɛ              2                        ⁢            A                              
Normally, the capacitance of the ridge capacitor CFR is far greater than the capacitance of the valley capacitors CFV.
Traditional TFT fingerprint sensor pixel circuits have only one transistor. In accordance with the operation of such structure, a slight difference in capacitance sensed at each pixel circuit must be read directly. However, it is not easy to achieve the required accuracy in difference. Some environmental conditions such as noise and cross talk will deteriorate the result. Therefore, it is necessary to find another solution to achieve accurate detection.